Optical fibers have been used in a wide sphere for optical communication, optical measurements and the like. For example, the optical communication tends to be used widely for broad band or LAN optical communication networks, photonic application in automotives, control of electric products and industrial appliances and the like. However, to construct optical communication networks with further increased capacity, long distance and highly functional operation, it is pointed out that conventional optical fibers forming waveguides called as cores by adding additives to quartz-based glass are limited in terms of optical properties. To deal with this issue, in these years, an optical fiber with a new structure having many air holes in its cross-section has drawn attention.
An optical fiber is generally composed of a core forming the center part where light is passed and a cladding forming the periphery of the core. The above-mentioned optical fiber with a new structure has a plurality of tubular air holes extended along the axial direction of the optical fiber in the core and/or the cladding and in the case where the optical fiber is cut, many air holes appear in the cross-section. As such the optical fiber, there are those having regularly arranged air holes in the cross section and those having irregularly arranged air holes. There are also those called as a holey fiber and as a photonic band gap fiber. Further, there exist those called as photonic crystal fiber. The name of the above-mentioned optical fiber with a new structure is not necessarily standardized, so that several names are used in multiple in some cases. Some of the names are, for example, index guiding photonic crystal fiber, air-clad fiber, hole-assisted fiber, photonic band gap fiber and the like, and although different in the air hole arrangement in the cross-sections, all of these fibers are called as a photonic crystal fiber. The photonic crystal fiber has regular arrangement of the air holes in the cross-section.
In this specification, it should be understood that the optical fiber having air holes in the inside, as described above for the optical fiber with a new structure, includes all kinds of optical fibers having a plurality of tubular air holes extended along the axial direction of the optical fibers in the core and/or the cladding and having a structure in which many air holes appear in the cross-sections in the case where the optical fibers are cut, and accordingly there are optical fibers with air holes arranged regularly in the cross-sections and optical fibers with air holes arranged irregularly in the cross-sections. It is pointed out that the optical fiber having air holes in the inside, particularly, the photonic crystal fiber, has the following characteristics which conventional optical fibers do not have: that is, the fiber makes possible a single mode at optional wavelength; has a high refractive index and flexural strength; has a large numerical apertures; and can be designed to have a refractive index and polarization characteristics as desired since the average refractive index is changed on the basis of the size and arrangement of the air holes.
However, in the case of attaching a connector for connecting the optical fiber having air holes in the inside, it is required to polish the end face of the fiber to make the surface flat, and if the polishing abrasive grains, polishing dust or the like enter the air holes at the time of polishing, not only the optical characteristics of the optical fiber are deteriorated, but also the polishing dust or the like is sometimes blown out of the air holes during its use to deteriorate the transmission property. With respect to a mechanical splice, in the case of directly butting and connecting the end faces of fibers after the fibers are cut, a refractive index matching agent (also working as an adhesive) is applied to the end faces and it is also required to avoid penetration of the refracting index matching agent into the air holes. Additionally, in the case of cutting the fibers, unlike conventional optical fibers, since the optical fiber has air holes, cracks may be formed from the air holes at the time of cutting. As described above, the optical fiber having air holes in the inside has a particular problem of the end face treatment which does not occur in the case of conventional optical fibers.
Since air holes are generally arranged in the cladding, a sealing composition for the air holes in an end portion is required to be an optical resin satisfying the following to give needed characteristics to the optical fiber: that the sealing composition has a lower refractive index than that of the core; the chargeability of the sealing composition in the air holes is high; the sealing composition is heat resistant in consideration of heat generation at the time of polishing; the polishing processability in the end face is good. Examples of the optical resin are a photo-polymerizable composition containing a specified epoxy-based fluorine compound (e.g. reference to Patent Document 1), a photo-polymerizable composition containing a specified acrylate-based fluorine compound (e.g. reference to Patent Document 2 and Patent Document 3). However, these compositions generally have a refractive index of 1.45 or higher and thus the refractive index is not low.
On the other hand, as the optical resin containing a fluorine compound having an acryl or epoxide group, there is, for example, an optical thin film using a cured substance of an epoxy compound having a fluorine-containing alkylene group (e.g. reference to Patent Document 4). This technique aims to improve the optical coating and although there is description that an optical thin film excellent in scratching resistance can be formed, the technique mainly relates to applications for reflection prevention films and is nothing to do with the sealing composition.
As described above, the connection issue, which is a characteristic problem of the optical fiber having air holes in the inside, does not exist in the case of conventional optical fibers. And since a technique of butting polished end faces of optical fibers and fixing the fibers in a connector is employed but not a technique of simply sticking a fiber and a connector with an adhesive, the problem is relevant to the end face treatment. To solve the problem, it is required to satisfy low viscosity, heat resistance, chargeability, polishing processability, and adhesion strength all in high levels beyond the minimum necessary levels, and sealing compositions which are satisfactory in these properties have not been made available yet.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-154233
Patent Document 2: JP-A No. 62-265248
Patent Document 3: JP-A No. 63-101409
Patent Document 4: JP-A No. 11-133207